This seemingly innocuous issue is in fact governed by an act of congress. Technically, we did pass some legislation some decades ago which required slow conversion to the metric system -- however, this was slowly changed in the ensuing years to become almost non existent.

You might wonder why? In several ways, this decision is more costly and political than you might at first believe. Along with the metric system of weights and measures, comes a host of other standards -- such as screw sizes, threading requirements, gearing pitches and other machine specific standards. Such standards have existed since before the turn of the century in the (at the time) most advanced countries in terms of manufacturing and standards: these were the US and Great Britain. Note Germany was known for cutting tools and the famous Solingen Steels (with inclusions of Tungsten Carbide-- but they didn't know it then) but they were not big on mass manufacturing with interchangeable parts-- a concept pioneered by Ely Whitney and his disciples. Also note that these countries were the major holdouts of the Metric system...

Here is why: re-tooling costs billions of dollars. It is a boon for the countries that already use the new standard and a bane for those who don't. A second issue is the quality of the standard -- ASME standard screws are based on steels available in the states during the mid 40's.At that time we had most advanced steel production in the world. The screws took advantage of the superior hardness and tensile strength so that the standard for high-end screws is lighter and smaller than their metric counterparts -- a disparity that largely remains true today.Similar issues occur in standard gears, although such gears have largely been replaced by modules. The current state of affairs is that most US,Chinese and Japanese companies have both sets of tooling in house and most production is slowly moving to metric standards and to newer joint standards that are being developed to fill the gaps in the metric machine standards.

There is a final issue -- that of measurement of the standards which must be traceable to the originals. Because of recent discoveries (quantum Hall effect and ion fountain clocks) most of the standards (i.e. length, voltage and such are now derived from others. Even the speed of light is now a defined constant, not a measured speed). One of the remaining constants is the kilogram (which looses mass every time it is handled). It is far easier and cheaper (and more repeatable and accurate) to compare with the American NIST standards than the international ones. If you are interested in the current state of defined standards and physical constants -- check out the APS (AmericanPhysical Society) website for the paper they update every year on physical constants.

Upshot is that it is a bit more involved than replacing street signs --which should have been done years ago -- if not for the difficulty in addresses and surveying...

Im with you; the United States is about 200 years behind the times in our system of measurement.People around the world use the metric system, but here in the US, most people, except scientists, have resisted it. The short answer is that Congress can make laws that tell businesses and agencies to use the metric system. Executive orders from the president can tell federal agencies to use it. I found an interesting article by David Smith of the Federal Highway Administration

He gives a brief history of the struggle to get the US on board with the metric system and concludes that it will happen soon, but you will notice that the article is dated 1995. Here we are almost 15 years later, and were still buying gallons of gas and driving miles of roadway.

There are some costs to converting industries, changing signs, and such, but the longer we drag our feet, the more that cost increases. In my opinion, the reason were still on an outmoded system is because many people are too lazy to learn and use the new system. Politicians dont want to be unpopular, so they choose their battles, only making the majority unhappy when they feel they have to. It is unfortunate because the metric system is actually much easier to learn and use. Teachers and students would be much better off if American students did not have to learn an old-fashioned, illogical system. Switching to the metric system now would be a gift to future generations of Americans.

The Office of Weights and Measures also has a useful document on the history of the Metric system in America:

It even has a recipe for chocolate chip cookies in metric.

Changing the unit system would require a law, which in turn would require the institution in the United States that passes laws, namely, the legislature: the House of Representatives, and the Senate. Of course, the President would have to *sign* such a bill before it could become law, but the law would be written by Congress.

So why don't we switch over to metric?
President Carter tried to push that - but the country - and the rest of the government - wouldn't listen, and as I said, a president alone doesn't have the power to make that kind of change.